For a number of years, medicament delivery devices have been developed, which contain a medicament container provided with a stopper arranged movable inside the container for delivering doses of medicament. The stopper is usually moved by an elongated plunger rod, the plunger rod is in turn often drivably connected to a force mechanism, such as spring force members, capable of accumulating energy for driving the plunger rod when released.
With many devices, a medicament container is placed inside one housing part of the device, while the drive mechanism, comprising the plunger rod, is arranged in another housing part. The device is then assembled in that the housing parts are connected to each other to make the device ready for delivery of a dose of medicament. Usually a proximal end of the plunger rod is, during the connection, brought in contact with a distally directed surface of the stopper. Before connection, the plunger rod is often brought manually to a position that theoretically will ensure a contact with the stopper after assembly. The same situation may also occur when a device has been used such that the medicament container has been emptied. When a new, full medicament container is to replace the empty container, the plunger rod is in its most proximal position and has to be brought to its initial position.
However, medicament containers tend to have quite a large tolerance range in the longitudinal direction, i.e. the actual length of a certain type of medicament container may vary within a span of several millimeters. Also the actual position of the stopper in the longitudinal direction of a new and unused medicament container may vary within quite a large span. These tolerance ranges may in total lead to a gap between the plunger rod and the stopper when the device has been assembled. In turn, this gap will lead to a delivered first dose that is smaller than the preset, intended dose because the plunger rod moves a certain distance during its delivery stroke without moving the stopper.
Because of the tolerance ranges, it is further difficult to provide a fixed stop for the plunger rod at the initial position, because either there will be a gap or the stopper will be pressed and deformed against the plunger rod, which may lead to difficulties in assembling the housing parts and/or that a large pressure is built up inside the medicament container or in the flexible stopper, which may lead to leakage or premature dose delivery.
US 2011/0245780 discloses a medicament delivery device comprising an adjustment mechanism for handling or adjusting an end surface of a plunger rod in relation to a stopper inside a medicament container. The adjustment mechanism comprises an adjustment member threadedly engaging an end of the plunger rod such that rotation of the adjustment member will cause it to move in the longitudinal direction in relation to the plunger rod, whereby an end surface of the engagement member will contact the stopper.
One drawback with the solution according to US 2011/0245780 is that it is difficult to know beforehand where to position the adjustment member in relation to the plunger rod for a certain medicament container. It would therefore be necessary to adjust and try the connection between the plunger rod and the medicament container a number times in order to eventually find the appropriate position with contact between the plunger rod and the stopper but without any force from the plunger rod on the stopper.
There is thus still a need for devices where it is ascertained that there is an initial positive contact between a plunger rod and a stopper of a medicament container before an initial dose delivery.